UNITED NATIONS —
On Tuesday, world leaders will convene in New York for their annual gathering at the United Nations General Assembly. The Syrian conflict - and an appearance by Iran's new president - are likely to overshadow the meetings.

This year, leaders will not meet in the tired grandeur of the General Assembly hall, which is undergoing a major renovation. Instead, they will gather for the annual debate in a spacious and modern conference hall that has been outfitted with the familiar green marble dais.

U.N. Secretary-General Ban Ki-moon has made clear that Syria will top the international agenda, saying it is the biggest peace, security and humanitarian challenge the U.N. faces.

“Let us be clear: the use of chemical weapons in Syria is only the tip of the iceberg. The suffering in Syria must end,” said Ban.

There will be meetings on the margins of the General Assembly between key players on Syria, including U.S. Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov, on how to implement a deal for Syria to give up and destroy its chemical arsenal.

Ban will meet with the foreign ministers of the five U.N. Security Council permanent members - Britain, China, France, Russia and the United States. He and his special representative on Syria, Lakhdar Brahimi, also plan to meet with Kerry and Lavrov, to discuss how they can build momentum toward a political solution of the Syrian crisis.

“So it is my sincere hope that, when we meet, we will be able to set a date for the Geneva II [Roman numeral 2] meeting.”

More than 100,000 people have been killed in Syria, and more than 6 million more have been internally displaced or become refugees.

An appearance by Iran's newly elected president, Hassan Rouhani, is drawing pre-meeting buzz.

Many leaders will be keen to hear what Rouhani has to say, and whether he is ready to improve relations with the West and answer outstanding questions about his country’s suspect nuclear program.

Speculation has been growing about a possible encounter between U.S. President Barack Obama and Iran's new head of state during the U.N. meetings. However, White House officials say there are currently no plans for the two men to meet.

Rouhani has been on a diplomatic charm offensive, meanwhile, offering interviews to American media and taking a far softer line than his predecessor, Mahmoud Ahmadinejad, who was widely vilified in the West for doubting the Holocaust and questioning Israel's right to exist.

Ahmadinejad's fiery rhetoric at the U.N. often was boycotted by Western and Israeli envoys who staged walkouts.

Obama is scheduled to be the second speaker at the General Assembly's opening session on Tuesday, after Brazilian President Dilma Rousseff.

Rousseff was to have made a state visit to Washington next month, but canceled plans for the trip abruptly this week, following disclosures that the U.S. has spied on the Brazilian government's internal communications. That could make any encounter between the two leaders awkward.

Other high-level meetings in and around the U.N. in the coming days will focus on Afghanistan, Egypt, Mali, the Central African Republic and the Democratic Republic of Congo.

On Thursday, Sudan’s President Omar Hassan Al-Bashir may appear at the podium. Since the International Criminal Court indicted him for war crimes and crimes against humanity in Darfur in 2009, and formally charged him with genocide the following year, his international travel has been curtailed.

The Sudanese president has indicated he hopes to attend the General Assembly debate. As the host country of the United Nations, the United States is obliged to grant him a visa to travel here. But if he does take the podium, that will outrage many members of the international community who have called for his arrest.

More than 130 heads of state or their representatives - one of the largest contingents in U.N. history - are scheduled to attend the General Assembly meetings.